Pursuant to U.S.C. xc2xa7119, this application claims the benefit of European Patent Application 02291502.9, filed Jun. 17, 2002.
The present invention relates to ophthalmic lenses and, more particularly, to a model for representing a complex surface of a lens, for example a progressive multifocal surface. It applies notably to the calculation of a prescription surface that is spherical or toric, of an ophthalmic lens.
Multifocal ophthalmic lenses are well known; from among multifocal lenses we can distinguish those known as progressive lenses, and lenses more specifically tailored to close vision. Progressive multifocal ophthalmic lenses are used for correcting presbyopic vision and allow the spectacle wearer to view objects over a wide range of distances, without removing his or her spectacles. Such lenses typically include a far vision region, located at the top of the lens, a close vision region in the lower part of the lens, an intermediate region connecting the near and far vision regions, with a main meridian of progression passing through these three regions. A reference point is provided for far vision in the far vision region, and a reference point for near vision is provided in the near vision region. Such lenses are for example described in French patent application FR-A-2,699,294, U.S. Pat. Nos. 5,270,745, 5,272,495, and French patent applications FR-A-2,683,642, FR-A-2,699,294 or FR-A-2,704,327.
Lenses that are more specifically dedicated to close vision also exist; such lenses do not have a far vision region with a defined reference point as do conventional progressive lenses. Such lenses are prescribed as a function of the power the wearer requires for close vision, independently of far vision power. This type of lens is described in an article in the April 1988 issue of the xe2x80x9cOpticien Lunetierxe2x80x9d and is marketed by the assignee under the name Essilor Interview. This lens is also described in French patent application FR-A-2,588,973.
Habitually, multifocal lenses, whether they be progressive or dedicated to close vision, include a non-spherical multifocal face, for example the face opposite the spectacle wearer, and a spherical or toric face, known as the prescription face. This spherical or toric face allows the lens to be adapted to the user""s ametropia as prescribed by an ophthalmic surgeon. A multifocal lens is thus generally a semi-finished product which needs to be adapted to the wearer by machining the prescription face. Such a multifocal lens can be defined by supplying hundreds of parameters defining the altitude of a large number of points on the surface. An adaptation process for a spherical or toric face is described in detail in European patent application EP-A-0 990 939; it is proposed to use, for defining the rear face, a ray tracing program and to proceed by optimization. In this case, the front face is modeled using Zernike polynomials. European patent application EP-A-0 990 939 also mentions that a non-spherical surface is generally defined by the altitudes of all the points on the surface, or yet again by values for mean sphere and cylinder at all points of the surface. Sphere and cylinder at a point being conventionally defined as the half sum and difference of the maximum and minimum curvatures of the surface at this point multiplied by a factor of (nxe2x88x921), where n is the refractive index of the lens material.
U.S. Pat. No. 5,444,503 proposes defining the lens prescriptions surface from aberrations over a whole lens surface, by appropriately varying a parameterized continuous surface, for example a surface defined by splines, using known mathematical optimization algorithms.
U.S. Pat. No. 6,089,713 corresponding to European patent application EP-A-0 857 993 discloses a lens having a spherical or non-spherical front face, with symmetry in rotation. The rear face is adapted to the wearer, to provide the lens with the sphere, astigmatism and prism prescribed, and their distribution over the lens. The multifocal surface is defined for each user.
U.S. Pat. No. 2,878,721 discloses a multifocal lens with a prescription face on the front of the lens. The prescription face is used for adapting the lens to the user, without the nature of this adaptation being however stated explicitly.
For single-focus lenses, power is conventionally calculated using the Gullstrand formula, at the optical center of the lens. It is consequently sufficient to know the refractive index n of the material, the radii of curvature of each face of the lens and their orientation, and thickness e at the optical center, to determine lens power. For example, if the front and rear face of the lens are locally spherical at the optical center, and C1 is the curvature of the front face and C2 that of the rear face at the optical center, Gullstrand""s formula is written, as the person skilled in the art knows, as:
P=(nxe2x88x921)(C1/(1xe2x88x92e(nxe2x88x921)C1/n)xe2x88x92C2) 
This approximation is in fact only valid when the point considered for the calculation has, locally, the shape of a sphere or torusxe2x80x94as is the case for a single focus lense, the local prism of the lens at the point considered is small and the radius for which lens power is calculated has zero angle of incidence with the latter.
Below we shall call xe2x80x9ccomplex lens for the point consideredxe2x80x9d or more simply xe2x80x9ccomplex lensxe2x80x9d, any ophthalmic lens for which the power at this point cannot be calculated by simple application of Gullstrand""s formula; complex lenses consequently comprise notably:
progressive multifocal lenses;
non-spherical single focus lenses,
spherical lenses having prism,
or more generally, any lens considered outside its optical center. Below, we mean by the term xe2x80x9ccomplex surfacexe2x80x9d of a xe2x80x9ccomplex lensxe2x80x9d any surface which is, overall, neither spherical nor toric.
Whatever the process used for providing the prescription face of a complex lens, the characteristics of the complex surface of the lens are required, it is consequently necessary to know the characteristics of a complex lens and to manipulate these characteristics for example, for supplying them to a processing machine.
One aim of the invention, in certain embodiments, is to model the complex surface of a lens, accurately and simply at one point or around one point, in order to be able to calculate power at this point, in different configurations, without it being necessary to know in an exhaustive fashion, the geometry of a complex surface of the lens.
The invention consequently provides a model for representing a complex surface of an ophthalmic lens, the model comprising:
a prism reference point and a prescription point;
the normal (or normal vector) to the complex surface at the prism reference point, and
the local characteristics of the complex surface at the prescription point or around the prescription point.
In one embodiment, the local characteristics around the prescription point comprise local characteristics within a circular patch or disc centered on the prescription point and of diameter greater than or equal to 2 mm.
It can also be arranged for the local characteristics around the prescription point to comprise local characteristics within a circular patch or disc centered on the prescription point and of diameter less than or equal to 12 mm. Alternatively, the local characteristics around the prescription point can comprise local characteristics within a circular patch or disc centered on the prescription point and not covering the prism reference point.
In all cases, the local characteristics at the prescription point can comprise the normal and the main curvatures with their orientations at this prescription point within a given reference frame.
The invention also provides model for representing a lens having a first surface and a second spherical or toric surface, the model comprising
a model of the first surface of the lens as set out above;
optical component thickness measured along an axis defined by the normal to the complex surface at the prism reference point;
the plane tangential to the second surface of the lens on the axis;
the main curvatures and their orientations for the second surface.
The invention further provides, in another embodiment, a method for calculating the prescription surface of a semi-finished lens having a surface is also provided, comprising:
providing a prescription comprising at least a power and a prism;
providing a representation of a surface in the model as set out above;
calculating a spherical or toric prescription surface using ray tracing passing through a prescription point or around the prescription point.
In this case, the calculation step can comprise:
choosing a lens thickness;
providing a starting prescription surface having, on an axis defined by the normal to the complex surface at the prism reference point, a prescribed prism;
varying curvatures and their orientation for the starting prescription surface, with constant prism, so that powers calculated by ray tracing approach prescribed powers.
Further characteristics and advantages of the invention will become more clear from the detailed description below of some embodiments thereof provided solely by way of example, and with reference to the attached drawings.